The present invention relates to the logging of earth formations, and more particularly to neutron irradiation measurement techniques for in situ determination of earth formation porosities in the vicinity of a borehole passing therethrough.
Present neutron methods for obtaining porosity measurements analyze secondary neutron populations produced by either steady-state or pulsed sources of high energy neutrons. Steady-state sources typically produce neutrons having mean energies of approximately 4-5 Mev. Pulsed (d,t) sources produce neutrons having energies of 14 Mev. In both, the neutrons are moderated by interaction with the nuclei of the materials in the borehole and the surrounding earth formations. When the neutron energies have moderated to below about 0.05 electron volts, they come into thermal equilibrium with their environmnt. At this point they are referred to as "thermal" neutrons and, on average, lose no further energy. At energies just above thermal up to around 100 electron volts, the neutrons are referred to as "epithermal" neutrons.
Upon reaching thermal energy range, the neutrons diffuse through the formation and borehole until they are captured by nuclei in the constituent materials. The rate at which a zone of material (formation or borehole) captures the neutrons (or more precisely, the probability of capture) is referred to as the macroscopic capture cross-section of the zone. The macroscopic capture cross-section is, in turn, a result of the combined microscopic capture cross-sections of the various constituent elements and materials constituting the zone. The capture cross-section of chlorine for thermal neutrons is considerably higher than that of most other elements commonly encountered in earth formations of interest. Accordingly, thermal neutron macroscopic capture cross-section measurements can give a good indication of the saline content of the fluids in the zone in question. By combining such information about the saline content of the fluids in the pore spaces of an adjacent earth formation with information about the formation water salinity, porosity measurements, and measurements of formation shaliness, information can be derived which can discriminate oil from salt water filled pore spaces in the vicinity of a well borehole.
Since thermal neutrons are absorbed by other materials as well as chlorine, the macroscopic capture cross-section is also responsive to borehole conditions and to the lithology of the formation materials. Present thermal neutron techniques for obtaining porosity measurements from pulsed neutron tools therefore often involve taking a ratio of the capture gamma ray count rates in two detectors spaced in the tool at different distances from the neutron source. By taking the ratio of the counts, many of these perturbations and some of the effects of borehole parameters can be reduced. However, to take such ratios the detectors must be gain stabilized, or at least their count rates must drift in exactly the same manner. Pulsed neutron measurement ratios thus obtained are therefore still borehole dependent, dependent on the capture cross-section of the formation, and can be dependent on the specific capture gamma ray energy distribution from the elements present (i.e., can be lithology dependent). In addition, the measurement is often statistically limited because the more distant detector used in the ratio measurement experiences a relatively low count rate in many formations. Nevertheless, a good qualitative porosity indication can often be derived.
Epithermal neutrons behave quite differently in the borehole and the formation than thermal neutrons. Epithermal neutron populations are depleted primarily through moderation by collisions (predominantly elastic) with formation nuclei, rather than by absorption. Epithermal neutron populations are therefore not affected by the various (i.e., borehole and formation) capture cross-sections. Instead, they are moderated and depleted most quickly by collisions with nuclei of similar mass: hydrogen. Since most pore space is filled by hydrogen-rich materials (hydrocarbons or water) while solid formation materials contain much less hydrogen, the rate of decay of an epithermal neutron population gives a good quantitative indication of the amount of hydrogen-containing material present. This, in turn, can be used to measure formation porosities. Further, such epithermal neutron measurements may be made without requiring two separate detectors. U.S. Pat. Nos. 4,097,737 and 4,266,126, for example, give examples of single detector epithermal neutron methods for measuring formation porosities while minimizing the effects of lithology.
However, prior art epithermal neutron porosity methods have typically suffered from several limitations. For example, borehole effects are commonly ignored. That is, since borehole moderation is usually faster than formation moderation, the several measurements which are made are usually delayed until the borehole component has had sufficient time to die away. However, some residual borehole component will usually be present, and neutrons can always be expected to reenter the borehole from the adjacent earth formation.
Another disadvantage of delaying the measurement gates to allow the borehole component to dissipate is the loss of important information during this long waiting period. Count rates are high and statistically important during early portions of the epithermal neutron moderation (or die-away) cycle, and such prior art delayed measurement methods fail to utilize this information. Further, as disclosed by the present invention, valuable information can be obtained even earlier from the nature of the epithermal neutron population build-up during and just following the pulse of high energy neutrons.
An important improvement in thermal (as distinguished from epithermal) neutron measurements is disclosed in U.S. patent application Ser. No. 383,680, filed June 1, 1982, now U.S. Pat. No. 4,409,481, and assigned to the assignee of the present invention, the disclosure of which is incorporated herein by reference. In this invention, at least four, and preferably six, capture gamma ray counts are made starting immediately after thermalization of the fast neutrons. From these measurements both the borehole thermal neutron lifetime component and the earth formation thermal neutron lifetime component are individually calculated using least squares fitting of the count rate data. Rather than ignoring the borehole component, therefore, it is specifically identified, calculated, and separated from the formation component, substantially improving the accuracy of the formation thermal neutron lifetime measurement. Unfortunately, however, the prior art fails to teach how such an improved method might be applied to the measurement of lithology-independent epithermal neutron populations.
A need therefore remains for an accurate, sensitive, versatile, single-detector epithermal neutron porosity measurement method which is essentially indifferent to formation lithology and borehole effects.